1. Field of the Invention
The present invention relates generally to air and fuel supply control systems for internal combustion engines, and more particularly, to a system for controlling air supply effected to an internal combustion engine by a plurality of superchargers accompanied with the internal combustion engine to be controlled to work sequentially and fuel supply effected to the internal combustion engine supercharged by one or more of the superchargers.
2. Description of the Prior Art
In the field of internal combustion engines employed in vehicles, there has been proposed a so-called sequentially controlled supercharging system in which a couple of turbosuperchargers of primary and secondary are provided for an internal combustion engine and so controlled that only the primary turbosupercharger works for supercharging the engine when intake air mass flow in an intake passage of the engine is relatively small and both the primary and the secondary turbosuperchargers work simultaneously for supercharging the engine when the intake air mass flow is relatively large, as disclosed in, for example, the Japanese patent applications published before examination under publication numbers 56-41417 and 59-160022. In such a system, an exhaust cutoff valve is disposed in a portion of an exhaust passage of the engine through which exhaust gas is applied to a turbine of the secondary turbosupercharger and an intake air cutoff valve is also disposed in a portion of the intake passage of the engine through which intake air compressed by a compressor of the secondary turbosupercharger is supplied to a combustion chamber of the engine, and each of the exhaust cutoff valve and the intake cutoff valve is controlled to be closed and open so as to cause the primary and secondary turbosuperchargers to work in the aforementioned manner.
In connection with the control in operation of each of the primary and secondary turbosuperchargers, it has been proposed to put the secondary turbosupercharger in a condition of preliminary rotation before the secondary turbosupercharger commences to work for supercharging the engine for the purpose of suppressing torque shock arising on the engine due to time lag in the starting characteristic of the secondary turbosupercharger. In such a case, it has been usual that the preliminary rotation of the secondary turbosupercharger is started by such a control as to open the exhaust cutoff valve for supplying the turbine of the secondary turbosupercharger with the exhaust gas under a condition in which the intake air cutoff valve is closed and an intake air relief valve which is provided in a bypass provided to the intake passage for detouring the compressor of the secondary turbosupercharger is opened, and caused to proceed under a condition in which the intake air relief valve comes to be closed before the intake air cutoff valve comes to be open. Accordingly, when the intake air cutoff valve is opened and the secondary turbosupercharger commences to work for supercharging the engine, intake air having stayed at a portion upstream to the intake air cutoff valve in the intake passage is suddenly forwarded toward the combustion chamber of the engine so that air mass flow which is larger than intake air mass flow detected by an air flow meter provided in the intake passage is practically supplied to the combustion chamber of the engine.
In the meantime, in a previously proposed fuel supply system employed in the engine equipped with the primary and secondary turbosuperchargers working in the manner mentioned above, quantity of fuel supplied to the combustion chamber of the engine is determined on the strength of a detection output of the air flow meter so as to be in proportion to the intake air mass flow detected by the air flow meter. This results in a disadvantage that the fuel supplied to the combustion chamber of the engine is caused to be deficient in relation to the air mass flow practically supplied to the engine so that an air to fuel ratio in the engine is undesirably changed when the intake air cutoff valve is opened and the secondary turbosupercharger commences to work for supercharging the engine.
Further, when the intake air cutoff valve is caused to be closed from open and the secondary turbosupercharger is terminated to work for supercharging the engine, the compressor of the secondary turbosupercharger is continuously rotated by means of inertia for a certain short period of time after the intake air cutoff valve is closed and therefore a part of intake air having passed through the air flow meter is applied to the compressor of the secondary turbosupercharger. However, the part of intake air having passed through the compressor of the secondary turbosupercharger is prevented from being forwarded toward the combustion chamber of the engine by the intake air cutoff valve which is closed, so that air mass flow which is smaller than intake air mass flow detected by the air flow meter is practically supplied to the combustion chamber of the engine.
In such an occasion also, quantity of fuel supplied to the combustion chamber of the engine is determined on the strength of a detection output of the air flow meter so as to be in proportion to the intake air mass flow detected by the air flow meter. This results in another disadvantage that the fuel supplied to the combustion chamber of the engine is caused to be in excess in relation to the air mass flow practically supplied to the engine so that an air to fuel ratio in the engine is undesirably changed during the short period of time after the intake air cutoff valve is closed and the secondary turbosupercharger is terminated to work for supercharging the engine.